The compound YBa2Cu3O7-x (YBCO) is presently one of only a few materials with significant potential for use in high-Tc superconducting (HTS) technological applications. The level of oxygen content determines the charge (hole) doping level in the compound and subsequently its electronic properties, including the critical temperature Tc, or the temperature at which the sample becomes superconducting. The temperature and partial oxygen pressure at which the sample is annealed determines the oxygen content of the sample.
In addition to the oxygen content, the electronic transport properties of epitaxial films of YBCO are influenced in large part by the YBCO structural properties—the YBCO defects, crystalline phases, and surface morphology—all of which can be inherently influenced by the deposition conditions and substrate properties. With respect to the substrate properties, the extent to which the lattice of the substrate matches that of YBCO strongly determines the quality of a thin film. However, due to the lack of a perfectly matched substrate, YBCO is commonly grown on various lattice-mismatched materials. But, even a lattice mismatch of no more than a few percent is sufficient to produce substantial stress in the film. This stress in the YBCO film can often be relieved by the formation of defects in the film.
As the film is cooled from its growth temperature, which can be typically between 700° C. to 850° C., additional stress in the film can be induced due to differing thermal expansion coefficients between the film and the substrate. During this cooling process YBCO also undergoes a structural phase transition from the tetragonal to orthorhombic phase. The restructuring of the YBCO lattice induces stress that is most effectively relieved by the formation of a-axis and b-axis domains that are separated by twin boundaries. This phenomenon can commonly be referred to as “twinning”.
The dimensions of the twin structures in films can vary considerably, with widths ranging from ˜50 nm to 800 nm, and lengths as long as 2 μm. The presence of the twin structures has the effect that bulk electronic transport measurements are effectively an average of the a-axis and b-axis conductivities. For most applications, this is not a major design consideration. However, in the case of micro/nano-scale structures, electronic transport properties become dependent on the locally dominant domain alignment, particularly in the case of quantum tunneling through a Josephson Junction. Therefore, for Josephson Junction applications, twinning can be an undesirable effect.
Single crystals of YBCO have been de-twinned under the simultaneous application of heat and uniaxial stress. This approach, however, has proved futile in thin films except for the special cases where a small portion of the film has been suspended above the underlying substrate. Mostly twin-free films have been grown on vicinal SrTiO3 substrates. However, a reduction of the superconducting critical temperature, Tc, has been observed for these films in addition to a dependence on variations of the local vicinal angle to the formation of other types of defects. Furthermore, a process to produce large numbers of Josephson Junctions in a single circuit with a film grown on vicinal cut substrates has yet to be demonstrated.
In view of the above, it is an object of the present invention to provide a YBCO film and methods for manufacture that allow for a manufacture of a YBCO film substantially without twinning. Another object of the present invention is to provide a YBCO film and methods for manufacture that result in an untwinned YBCO film, but without reducing Tc. Still another object of the present invention to provide a large scale YBCO film and methods of manufacture, which result in a YBCO film having sufficient crystallinity that the YBCO film can be used for Josephson Junctions with controlled tunneling direction in the a-b plane to exploit the anisotropic d-wave symmetry of the superconducting order parameter. Another object of the present invention to provide a YBCO film and methods for manufacture which can be easy to implement in a cost-effective manner.